with the frequency of posts about searching for and fixing vacuum leaks for our cars, why hasn't some troubleshooting technique using a vacuum gauge come up? i have an old fuel pressure (range for carbed engines)/vacuum gauge i got along time ago and i know i've seen troubleshooting in repair manuals (Lash for one...) i hooked it up "t-ed" off the fuel pressure regulator (74 L-jet) but honestly don't know what i'm looking for/at. with concern about fixing vacuum leaks, wouldn't the gauge be the first indicator if vacuum is not what it should be at idle, for example? what should it be at idle?

Unmetered air the the problem. Vacuum leak is the difference between metered air and actual, used-for-combustion air. I think your idea is a good one in theory, but to find a way to measure the difference between what the ecu sees and actual is a tough nut to crack. Not to sound like a smart answer, but if it could be done in a practical manner Mr. Bosch would have loved to know how. It's a valuable piece of information that would allow the ecu to figure air/fuel ratio with highest precison.

I'm sure there's good insight from Mr.'s Anders, Darling, and others on the subject. I hope they chime in!

Unmetered air is a problem with L-Jet, but not necessarially with D-Jet. Air leaks in D-Jet cause a high idle condition, but the mixture is correct. Air leaks in L-Jet cause a lean mixture condition, causing idle and part-load drivability problems.

The problem with using a vacuum gauge for detecting air leaks is twofold. First, what is the "normal" vacuum level at idle? It varies from 14 to 18 in. Hg, depending on the condition of the motor and its temperature. Non-stock cams drop it lower. Secondly, let's say you do know what the "normal" level is, and you measure a different value. What have you learned? Not much that's useful. What you need to do is to find the source of a leak.

Various ways to do this have been suggested. Most involve spraying a combustable around suspected areas and noting whether the engine speed changes. Carb cleaner, starting fluid, propane, etc. have been suggested. This method locates the leak location quickly.